1. Field of the Invention
The present invention relates to a process for the separation of krypton from a radioactive waste gas mixture which is released during the chemical dissolution of burned-down nuclear fuel particles, and which in a carrier gas such as air, in addition to krypton, also contains xenon, argon, nitrous oxide and residual gas components. The waste gas mixture flowing out from a resolver has xenon removed therefrom after purifying the waste gas of nitrous oxides and radioactive residual gas components, such as aerosols, iodine, tritium and carbon (C-14) dioxide; and wherein krypton is finally withdrawn from the residual gas mixture and then stored. The invention also relates to an arrangement for the implementation of the inventive process.
During the reconditioning of fuel elements, the nuclear fuel particles are chemically dissolved in order to separate out the fission products or their decomposition products which are produced during the reactor operation from the renewed fuel and/or breeder materials which are reusable for the production of fuel elements. The resolver has air conveyed thereto for the oxidation of the nuclear fuel materials. The air quantity which is introduced for the scavenging of the resolver is so adjusted that possibly all volatile radioactive harmful materials are picked up and conveyed away. For example, for a resolver having a 500 kg/h uranium throughput there must be calculated a scavenging air quantity requirement of 120 Nm.sup.3 /h.
2. Discussion of the Prior Art
Essentially three methods are currently known for the separation of krypton from the waste gas mixture: (a) low-temperature rectification after liquefying at least a portion of the waste gas mixture, (b) absorption of the krypton in suitable solvents, (c) adsorption on activated charcoal or molecular strainers.
Low-temperature rectification provides the advantage in that there can be achieved a high degree of enrichment, in effect, purity of the final products; in contrast therewith, a disadvantage resides in the high technological requirements, particularly with consideration given to the adequate operational safety of the installations as well as the considerable energy requirement; referring, for example, to German Pat. No. 24 26 764.
In order to avoid these risks and costs, the absorption or the adsorption of krypton is of significance. An absorption of krypton in solvents is described by Merriman et al., "Removal of noble gases by selective absorption", International Symposium on Management of Gaseous Wastes from Nuclear Facilities, Vienna, 1980. Freon is utilized as the solvent. For the adsorptive separation, from D. T. Pence et al. "Noble gas separation from nuclear effluents using selective adsorption with inorganic absorbents", 16th DOE Nuclear Air Cleaning Conference, San Diego, 1980, it is known to remove in sequential steps from the waste gas the components in the following sequence: H.sub.2 O, CO.sub.2, Xe, O.sub.2, Kr from an N.sub.2 carrier gas stream in adsorption columns. The krypton separation is hereby effectuated in a quasi three-step mode, wherein the krypton is frozen out during one of the process steps. With regard to a further adsorptive noble gas separation, in particular the separation of xenon from the waste gas mixture, a discussion is set forth by H. Jungten et al., "Versuche zur adsorptiven Abtrennung von Edelgasen aus den Abgas einer Wiederaufarbeitungsanlage", 1978, pages 415 through 456, as well as in German Pat. No. 22 10 264. In accordance therewith, required for a waste gas throughput of 100 m.sup.3 /h are approximately 8t of activated charcoal in order to remove the xenon and to enrich krypton in the remaining waste gas to a 25-fold concentration.